The present invention relates to a proton (H+) conducting electrode, a method for its preparation and to an electro-chemical device employing a proton conducting electrode.
A variety of electro-chemical devices, constructed on the basis of a reaction of decomposition of a hydrogen gas, or a chemical substance containing hydrogen atoms, on an electrode to yield protons (H+) and electrons, a reaction of protons and electrons to yield hydrogen, or a reaction of protons, electrons and oxygen or another substance to yield water or a further substance, such as fuel batteries or other proton-type batteries, or chemical sensors, have been proposed.
Since electrons, protons and other substances, such as hydrogen gas, oxygen gas or water, are involved in the above various reactions, the site where all of these substances meet together represents a sole site of reaction.
For example, if a catalyst exhibiting electronic conductivity is supplied in a more or less dispersed state onto the surface of a proton conductor, the contact point between the protonic conductor and electrons and in its vicinity represent a site where protons, electrons and other gaseous substances can exist together. In general, such site is termed a three-phase interface.
FIG. 1 shows a prior-art example of an electrode structure. In the electrode structure, shown in FIG. 1, a catalyst 3 exhibiting electronic conductivity is dispersed on the surface of a proton conductor 1, with the surface of the catalyst being then covered by a gas transmitting current collector 5. If only the surface of the proton conductor 1, with the catalyst 3 dispersed thereon, is used for the reaction yielding the protons (H+) and electrons, a three-phase interface 7 is present in the vicinity of contact points of the proton conductor 1 and the catalyst 3. However, the site where all of electrons (e−) 4, protons (H+) 8 and gases 6, such as hydrogen gas or oxygen gas, meet together, is limited to a point-like area, this point-like area serving as a sole three-phase interface, with the result that the function as the electrode is not manifested satisfactorily.
Currently, for improving the function as an electrode, such a technique is used which consists in mixing proton-conducting components into an electrode material for forming a three-phase interface on the surface of the protonic conductor throughout the entire electrode formed to a certain thickness.
With this technique, electronic conducting paths are formed in a meshed pattern within the electrode by the catalyst itself or by an electrically conductive assistant material specifically mixed into the electrode material, whilst the proton conductor contained is also formed in a meshed pattern. If the other component than protons and electrons is a gas, the electrode itself is to be porous to allow the gas to be permeated throughout the electrode. If the other component is not a gas but a solid phase, the solid phase is added to the electrode. In any of these cases, the three-phase interface is to be formed over the entire electrode, as described above, to provide for as many reaction points as possible to improve the function as the electrode.
It is noted that, in the above electrode operating at a temperature lower than 100° C., inclusive of the ambient temperature, a proton dissociating liquid or a proton conducting high-polymer solid electrolyte, such as Nafion, manufactured by Du Pont, de Nemur, is currently used as the protonic conductor mixed into the electrode. In particular, with the use of Nafion, the device may be solidified, and hence may find extensive application. Thus, the device tends to be used extensively as a fuel battery for low temperature.
However, Nafion, which is a solid material, suffers a problem that, by reason of its proton conduction mechanism, its proton conduction performance is not displayed except if the Nafion itself is soaked with a sufficient amount of water. Thus, if Nafion is contained in an electrode, the device is difficult to use continuously under a dry atmosphere.
A need therefore exists to provide an improved proton conducting electrode that can be readily made and effectively used.